Even though Bangladesh logged 15,61,463 Covid-19 cases as of Friday, experts believe around 70 percent population of the country may have contracted the virus and developed natural antibodies.

Some experts think that the country is close to reaching the herd immunity threshold while some others say a comprehensive serosurveillance report is necessary to reach a conclusion over it.

They all, however, viewed that it is necessary to inoculate around 70-80 percent of people of the country to attain herd protection against the Coronavirus as any virulent new variant like Delta can emerge and dodge people’s natural immune protection.

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Herd immunity is a concept based on the body's immune resistance to the spread of a deadly disease (bacterial or viral infection) and it can be obtained in two ways -- naturally through infections of the majority of the population and artificially through vaccinating around 80 percent of the population of a country.

Official data on infection, vaccination

As per the data of the Directorate General of Health Services (DGHS), Bangladesh reported 15,61,463 cases till Friday while the positivity rate was 2.77. The daily-case positivity rate in the country remained below 5 percent for over last two weeks.

According to the Directorate General of Health Services (DGHS), a total of 35,829,418 people received the first dose of vaccines while 17,795,120 ones both doses as of October 7. The country has so far collected 64,568,420 doses AstraZeneca , Pfizer, Moderna and Sinopharm vaccines.

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Meanwhile, Health Minister Zahid Maleque on Saturday said around five crore people have already been vaccinated while 50 percent will be brought under the vaccination by December next.

He said they also have a plan to vaccinate 70-80 percent people of the country by March next year.

Debate over antibody, herd immunity

Based on some studies on samples collected from both symptomatic and asymptomatic Covid positive individuals, Ramon Magsaysay Award-winning Bangladeshi scientist Dr Firdausi Qadri said 60-70 percent people of Bangladesh have gained some types of antibodies against Covid-19.

Read: Bangladesh unlikely to reach Covid herd immunity anytime soon: Expert

She said the Covid infection rate declined significantly in the country due to the development of Covid-19 resistant herd immunity.

Dr Qadri, an emeritus scientist at icddr,b, however, said herd immunity is not only dependent on infection-related exposure and seroprevalence as it is also dependent on vaccination of at least 70% population. “So, with the exposure and infection-related seropositivity, vaccination status and rates, in a country are related to herd immunity and herd protection,” she observed.

Renowned scientist and Gono Bishwabidyala’s microbiology department Prof Dr Bijon Kumar Sil thinks Bangladesh has almost reached the herd immunity threshold against Covid through natural infection.

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“Though Dr Firdausi Qadri said 60-70 percent people gained antibodies, I personally think it’s over 80 percent. Otherwise, the Delta variant wouldn’t have come under control. But if we can carry out any scientific research with representative samples, then we’ll get a clear idea about the percentage of our population with antibodies,” he said.

Dr Bijon said it is not possible to contain a virulent variant like Delta without antibodies among the majority of people. “As the current Covid infection rate has come down to below three percent, it’s an indication that over 80 percent people have now antibodies in Bangladesh.”

Even though, he said vaccination is necessary to boost up the antibody as it declines after a few months. “If we inoculate and at least our 80 percent people, then we may not witness any major outbreak of the virus even if any new variant like the Delta one emerged.”

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Prof Muzaherul Huq, a former adviser to WHO South-East Asia region, said Bangladesh is moving towards herd immunity. “But we still couldn’t attain the herd immunity though many experts believed immunity developed among around 60-70 percent people through contracting the virus.”

He said the actual herd immunity can be achieved only by vaccinating 80 percent of the population.

Public health expert MH Chowdhury (Lenin), chairman of the medicine department at the Health and Hope Hospital, said there is no available scientific evidence and serosurveillance report that suggests Bangladesh reached the heard immunity against Covid.

Read: Bangladesh far away from herd immunity; only massive vaccination can help: Experts

“We shouldn’t focus on herd immunity now. As the virus now remains under control, we should make a target to eliminate it by vaccinating most of the population as early as possible,” he said.

Natural infection-induced antibodies stronger

Dr Qadri said natural infection usually is very strong and the more severe the disease the greater the immunological response. “Vaccination is, however, dependent on the specific type and formulation of vaccine and can be different. Together the response can be higher.”

Dr Rubhana Raqib, another senior scientist at icddr,b, said the antibody developed through Covid infection may not last long. “Research has shown that the antibody lasts for 3-4 months in some people while it lasts around 9-10 months in others.  As the levels of protective antibodies in people wane after a few months, it can be boosted again with vaccination.”

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She said immunity from vaccines will give some protection against the new variants, but natural immunity may not be able to do it. “Virus has many antigens, but the vaccines are developed targeting some specific antigens for creating antibodies against those. When the virus changes its character, the antibody can’t identify and fight the new antigens, but it can neutralise the old antigens and thus the antibody can reduce the severity of the disease.”

Asked which antibody, the natural one or vaccine-generated one, is stronger, Rubhana said scientific studies have shown that the response of antibodies from vaccines is higher than the antibodies from infections.”

She said some vaccines, such as Pfizer and Moderna, have been developed considering some components of the virus while Sinopharm developed it considering the whole inactivated virus. “So, these two types of vaccines may give different levels of protection.”

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Dr Lenin said vaccines produce neutralising antibodies that stay for a longer period than the natural antibodies gained through Covid exposure.

“Some experts believe that neutralising antibody regarding Covid is much stronger than the natural antibody. That’s why no country in the world is counting on natural immunity as all are trying to get herd protection through inoculating around 80 percent of their populations,” he observed.